The same composition as perlite but in larger chunks, Sponge Rock is a naturally occurring, mixed glassy silicate of volcanic origin which 'pops' when furnaced. Its unique internal structure consists of numerous tiny closed air cells surrounded by a large exterior irregular surface. Sponge Rock is ultra-lightweight, inert, sterile, permanent, incombustible, asbestos- free, non-toxic, rot and vermin proof and has a neutral pH. Its extremely large, open surface area allows it to retain water in mixes while maintaining air spaces, important for healthy root growth.
We use Sponge Rock as part of our Medium Orchid Mix.